In a typical printing unit the printed product is discharged from the printing unit and typically is directed downstream through a dryer unit, followed by a chill unit. Problems occur when a web break occurs downstream of the press such as in the dryer or chill unit.
Should there be a web break there is a possibility that the web will be directed back to the printing press where it will become entangled in the press rolls. More particularly, the end of the printing web may wrap around the blanket cylinder damaging the blanket and resulting in substantial down time and blanket expense. This problem is particularly acute when the operator is not even aware of the web break.
The prior art has suggested several solutions to the problem. One proposed solution is to provide sensor means for detecting web breaks within the dryer and/or chill units. The detectors may be of the air, infra-red, photoelectric, contact, ultrasonic or other type capable of detecting a web break and signalling such information to a circuit which would cause the press to stop its operation. However, the press may be running at speeds of 1,000 to 3,000 feet per minute. The press rolls do not stop immediately but periodically decelerate over a period of time perhaps ten seconds. During this period of time, substantial printed product can be emitted from the press causing damage to the blankets by wrapping in the printing units.
In order to alleviate the problem some prior art devices utilize a web break detector which not only provides a signal which stops the press but also activates knife means or other severing device which severs the Web. Preferably, the web severing device is located before the dryer so as to reduce the amount of untensioned web. But since there is a period of time before the press stops, there remains the possibility that the untensioned web will go back to the press units and be wrapped around the blanket cylinders.
In prior art web break detectors, the web break is detected mechanically, optically or electrically which activates a shearing mechanism downstream of the printing unit and shuts down the printing unit to prevent additional printed product from issuing from the printing press. During the time period between the web breaks and the press shut down, the paper printed product will continue to issue. However, since there is no tension in the printed product due to the web break, the paper web will become entangled with the blanket cylinder and impression cylinder and may wrap around the blanket in the blanket cylinder. Typically, the blanket on the blanket cylinder is fragile in nature and may easily be compressed or otherwise damaged. When this occurs the blanket may have to be replaced, which can be expensive. The blanket cylinder is also somewhat inaccessible. Accordingly, it is a time consuming, complicated task to remove the wound printed web from the blanket and associated cylinders and after this is done the blanket must be removed and then replaced by a new, fresh blanket. All of these steps are time consuming and will result in substantial press down time.
There have been numerous attempts to overcome the problems associated with the detection of a breakage of printed product during the printing process.
Another proposed solution eliminates the necessity for web severing device. One such prior art attempt is shown and described in U.S. Pat. No. 4,846,060 to Proctor entitled "Printing Web Tensioning System." This prior art practice involves the use of at least a pair of rollers sometimes called anti-wrap rollers which are positioned downstream of the printing unit and immediately before the drying units. These rollers are normally positioned one on the upper and one on the lower side of the printing web. In normal operation, the rollers are spaced apart and not in contact with the web. Upon detection of a web disturbance indicating a likelihood of or actual web break, a mechanism is activated which causes the rollers on opposite sides of the web to come together with the printed product between the rolls along the width of the web. The rollers are rotated at a synchronized speed with respect to the speed of the printing cylinders. In this way, tension is supposed to be maintained on the web and the web will not be wrapped around and become entangled with the printing rollers. Although a certain amount of paper will be wasted as it is directed to the floor, such waste is minimal as compared to the problems, waste and expense associated with wrap around the blanket cylinders.
Another prior art proposal is shown and described in U.S. Pat. No. 4,549,485 entitled Paper Web Seizing Apparatus For Use With Printing Machinery. In this prior art device, rollers are provided so as to cause the broken web to wrap around rollers which are positioned downstream of the printing press.
Prior art devices made in accordance with U.S. Pat. No. 4,846,060 and modified versions thereof, have met with a certain amount of success in the commercial field when used at relatively low press speeds such as not in excess of 1200-1600 feet per minute.
One problem that has arisen with certain prior art devices is the perceived necessity for speed synchronism between the last printing rolls and the anti-wrap rolls so that the surface speed of these rolls is identical. It is very difficult to obtain the same surface speed for the last press rolls and the anti-wrap rolls for a variety of reasons such as problems associated with gearing, motors and the diameter of the respective rollers as well as diameter changes that occur from wrapping the web on the anti-wrap rollers.
If the surface speed of the anti-wrap rolls is slower than the surface 1600 of the press roll slack may occur in the area between the press rolls and the anti-wrap rolls. If a sufficient build up of slack occurs, the slack may work back to the press rolls and become wrapped around the blanket. Alternatively, if the surface speed of the anti-wrap rolls is too fast relative to the press rolls, excessive tension may result causing the web to break, causing the end of the web to wrap around the blanket. This may occur regardless of the press speed.
In addition, significant problems arise when the press speeds are increased above about 1600 feet per minute. In particular problems arise when the press speed is raised to about 3000 feet per minute. At these press speeds it has been found that it takes at least about seven seconds for a press to decelerate to a stop position after web detectors have signalled a press stoppage. During the time interval between the web break and the actual stopping of the press rolls it is possible that about 300 feet of paper will be fed through the last press feeding rollers. Without the anti-wrap rollers, the press rollers will become excessively wrapped and entangled with the printing product emitted from the press after the web breakage.
Additional problems may occur where the press is equipped with anti-wrap mechanisms such as in U.S. Pat. Nos. 4,846,060 and/or 4,549,485. The problem is that the diameter of these rolls will increase when the anti-wrap rolls thereafter are wrapped at a rate which will cause the roller speed to decrease and prevent all the excess printed web product from being taken up by such rollers. When this occurs the excess printed web product will be fed back to the printing press will cause the same problem of wrap around of the blanket cylinder as occurs without anti-wrap rollers.
Although the problems to which this invention is directed is not limited to breakage occurring in presses having dryer units, the breakage is more likely to occur in such an environment due to the severe temperature differentials.